ImagePreviewOutputSettings.format property

The file format the stills are written in. Defaults to 'jpeg'.

This is really a choice about the ALPHA CHANNEL rather than the container: JPEG cannot carry alpha, so a still of a source with transparency arrives with its transparent pixels flattened to black. That is invisible when the still is viewed on its own, and wrong as soon as one still is stacked over another (for example composing thumbnails into a layout diagram). 'png' keeps the alpha, at the cost of a larger, lossless file.

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